The present invention relates to preparation of monocrystalline compound semiconductors, particularly gallium arsenide.
To successfully grow large single crystals of GaAs by the liquid encapsulated Czochralski (LEC) technique, the GaAs starting material must first be compounded from elemental gallium and arsenic. The purest and most cost effective way to do this is to compound at low pressure in the crystal puller just prior to growth. See Pekarek, "Apparatus for Synthesis of Gallium Arsenide Under Liquid Encapsulant", Czech. J. Phys. B20 (1970), page 857, which is hereby incorporated by reference. The way this has been done at Texas Instruments is shown in FIG. 1. A radiatively heated quartz cell is used to force arsenic vapor through an injection tube into the liquid gallium melt, where it reacts chemically to form GaAs. The approach shown in the figure is very similar to that used by others in the industry, most notably Hewlett-Packard. See Puttbach et al, "Liquid Encapsulated Synthesis and Czochralski Growth of GaAs at Low Pressure", Fifth International Conference on Vapor Growth and Epitaxy (Coronado 1981), Abstracts p. 75. The disadvantage to this approach is that the pressure inside of the cell tends to drop below that outside of the cell at various times during the compounding cycle, and this can cause the liquid Ga and GaAs to be sucked up into the cell, ruining the run and creating a safety hazard. This can be avoided by using very high flux of arsenic, e.g. by externally heating the arsenic cell. However, a large flux of arsenic has two major disadvantages: first, the resulting high condensation of arsenic in the furnace chamber means that growth cannot occur in the same chamber where compounding has been done. This means that a transfer step is necessary, with large possibilities for contamination of the semiconductor material. Secondly, a very rapid introduction of the arsenic leads to poor control of melt composition, which leads to seriously degraded control over the crystal-growth process. Control is degraded since an unknown amount of escaped arsenic vapor will enter the chamber atmosphere and condense on the colder parts of the furnace chamber walls. Control of melt composition is important, since the melt must be about 2% arsenic-rich (or have a slightly larger excess of arsenic), if a stoichiometric crystal is to be pulled: if the melt is very much richer in arsenic, e.g. 5% excess arsenic or more, the crystal which is sought to be pulled will twin, or cannot be seeded, or will be pulled as polycrystalline material. If the melt has less than 2% atomic of arsenic, the crystals pulled will tend to be nonstoichiometric, and have a high concentration of arsenic vacancies. Thus, while the use of very high arsenic flux rate solves the suction problem, it also severly degrades control of the process generally.
Thus it is an object of the present invention to provide a method for compounding gallium arsenide in-situ immediately prior to crystal growth, in which good control of the melt composition is retained.
It is a further object of the present invention to provide a method for compounding gallium arsenide in-situ immediately prior to crystal growth, without introducing a large amount of arsenic into the chamber atmosphere.
It is a further object of the present invention to provide a method for compounding and growing semiconductor-grade gallium arsenide crystals, without requiring the compounded gallium arsenide to be exposed to the atmosphere prior to crystal growth.
The present invention has alleviated the suction problem by adding a stabilization valve to the cell as shown in FIG. 2. This valve prevents the pressure inside the cell from ever becoming less than that outside.
Thus, the primary object of the present invention is to permit in-situ compounding of gallium arsenide, as a preliminary to pulling a crystal of gallium arsenide, without the risk of introducing molten gallium or gallium arsenide into the arsenic chamber.
Introduction of the hot melt into the arsenic chamber is dangerous because the melt is many hundreds of degrees above the sublimation temperature of arsenic. Thus, essentially all the arsenic in the quartz cell will immediately vaporize. This rapid thermal vaporization is very likely to break the quartz arsenic cell, and dump the melt which has flowed up into the cell back down into the remaining melt in the crucible. The result of this is that gallium and gallium arsenide will be splashed all over the chamber, causing significant damage to the apparatus and necessitating a lengthy cleanup. Moreover, the sudden vaporization of much of the arsenic charge may even bring the internal pressure in the puller high enough that some arsenic vapor escapes into the atmosphere. Such arsenic vapor will immediately react to form arsenic trioxide dust, which is extremely poisonous.
According to the present invention there is provided: a system for compounding gallium arsenide, comprising: crucible means for holding a melt of gallium and gallium arsenide; an arsenic sublimation cell, said sublimation cell comprising an arsenic vapor injection tube downwardly extending from an upper portion of said arsenic cell; means for positioning said arsenic cell so that said arsenic vapor injection tube extends into the interior of said crucible; wherein said arsenic cell further comprises a stabilizing valve, said stabilizing valve comprising a ball check valve to prevent low relative pressure inside said arsenic cell.